Combined combustion furnaces and heat exchangers, commonly known as process preheaters are used in many industrial applications to heat process fluids, particularly process gases such as air and sulfur dioxide used in the manufacture of sulfuric acid. The preheaters may be used intermittently or continuously. Conventional preheater systems have included horizontally or vertically aligned furnaces which bum fossil fuels such as natural gas or various grades of fuel oils. The heat exchangers have included vertically or horizontally aligned exchangers wherein heat transfer to the process fluid from the furnace gas occurs. Typically, the flow of the furnace gas is countercurrent to the flow of the process gas to enhance transfer of energy and, thus, improve efficiency.
In the manufacture of sulfuric acid, older preheater systems generally comprised a furnace and an associated heat exchanger wherein the furnace was formed of a brick-lined cylindrical shell having an air blower wherein the heated furnace gas exited from the end remote from the air intake and blower. Such fossil fuel combustion furnaces produced a flame extending as much as 3-4 metres in the furnace and only modest efforts were expended to efficiently mix fuel and air. Such furnaces generally required significant periods of time to heat the brick lining to operating temperatures, which brick preheating time affected the operation of the downstream plant.
Such heat exchangers were initially formed of carbon steel, which limited the temperatures that could be generated in the furnace to less than 650.degree. C. Further, these exchangers generally had their heat exchanger tubes vertically aligned and received furnace gas therethrough, while the shell space received the process gas to be heated. These carbon steel exchangers were susceptible to high temperature scaling and, thus, needed to be frequently replaced. In addition, in consequence of the very high temperatures produced in the furnace, it was necessary for large quantities of excess air and/or, larger exchangers to be used. High temperature combustion further increased the risks of formation of unwanted nitrogen oxides and smoke in the preheater exit gas.
Later preheater exchangers were formed of stainless steel and were, thus, able to operate at higher temperatures to provide higher thermal efficiencies. In the sulfuric acid industry, the preheater systems generally had long, horizontal, cylindrical furnaces with either a vertical exchanger or a horizontal exchanger mounted on top of the horizontal furnace. These newer designs also permitted rapid firing in the furnace, incorporated flue gas recycle and air preheating where required to improve thermal efficiency and to minimize formation of nitrogen oxides.
Preheater systems presently in use suffer from a number of disadvantages. It has been found that the shape of the combustion flame of the furnace may be variable in operation and cause inefficient radiative transfer of heat to the heat exchanger. Relatively low intensity combustion results in a longer residence time of the reactants in the furnace which favours the formation of unwanted nitrogen oxides. Further, high temperatures of the metal at the hot end of an exchanger may cause high temperature damage by scale formation and uneven thermal stresses. Yet further, most preheaters of the prior art do not allow of easy adaptation to higher energy efficiency by such optional features such as stack gas recycle and air preheating with stack gas. Horizontal, cylindrical furnaces occupy significant space on industrial sites where space is at an economic cost premium.
Accordingly, there is a need for an improved preheater system which does not suffer from the foresaid disadvantages of prior art preheaters.